1. Field of the Invention
The present invention relates to a magnetic encoder using a magnetic sensor with a spin-valve type giant magneto-resistance effect film.
2. Description of the Related Art
In recent years, a magnetic encoder applied to civil appliances, such as small-sized robots, digital cameras and ink-jet printers, is required not only to be cheap and small-sized but also to have a high resolution and excellent gap output characteristics. In other words, it is required that a magnetic encoder is small-sized but does not need a processing circuit for doubling frequency of signal and also can maintain a stable output against gap variation during its operation. Further, low electric power consumption is required.
In a conventional magnetic encoder, magnetic resistors formed from an anisotropic magneto-resistance effect film (hereinafter referred to as “AMR element”) are applied. The AMR elements are widely used, because electric resistance is changed by some percent by a magnetic field change in them even in a region of a relatively small magnetic field, and because their films are easily produced. However, it is necessary to thicken the film to 20 nm to 40 nm, in order to obtain a stable magneto-resistance effect for the AMR elements having NiFe alloy thin film or NiCo alloy thin film. But, they are difficult to use, since electric resistance of the elements reduces due to the thicker film. If a width dimension of the AMR element is reduced to increase the resolution, a shape anisotropy (Hk) is increased together with an influence of the thicker film and a sufficient electric resistance change cannot be obtained in a region of a weak magnetic field to cause an expected electric output. Because of the reason, it was difficult to enhance the resolution in a magnetic encoder using AMR elements. Increase of the resolution means narrowing pitches of the elements and/or magnetizations on a medium and also increase of a number of the electric output signals for a unit length.
Instead of the AMR elements, which are difficult to enhance resolution, an element using a coupled giant magneto-resistance effect film (hereinafter referred to as “coupled GMR element”) is disclosed in Japanese Patent 2812042. The coupled GMR element has an electric resistance variation ratio as much as twice to four times of the AMR element. In the coupled GMR elements described in Japanese Patent 2812042, an artificial lattice metallic film having some-ten layers of alternately laminated NiCoFe thin films and non-magnetic metal thin films is used. Multiple lamination of the ferro-magnetic thin films and the non-magnetic metal thin films leads to a large magneto-resistance variation ratio. However, it is difficult to accomplish a low electric power consumption, since the non-magnetic metal thin films are a good electrically conductive material and the electric resistance of the film is as low as a half to a third of that of an AMR element. The coupled GMR elements have an electric resistance variation ratio as much as 20% to 30%, but the electric resistance variation ratio can be obtained only by using them in a large magnetic field. By the reason, it was hard to use them in a relatively small magnetic field as in a magnetic encoder.
There is a spin-valve type giant magneto-resistance effect film as used in a magnetic head of a hard disk storage device (HDD), which is a film showing, in a region of a relatively small magnetic field, an electric resistance variation ratio as much as a coupled GMR element. As described in Japanese Patent 3040750, the spin-valve type giant magneto-resistance effect film is composed of a pinned magnetization layer, in which a magnetization direction is not changed by a variation on an external magnetic field (or magnetic flux) direction, a non-magnetic conductive layer and a free magnetization layer, in which a magnetization direction is changed following a variation of an external magnetic field. An element machined from a spin-valve type giant magneto-resistance effect film (hereinafter referred to as “SVGMR element”) has an electric resistance as large as five times to six times of that of the coupled GMR element, and reduction of electric power consumption is easily achieved when it is used for a magnetic sensor. Also, it can work in a region of a magnetic field as relatively small as 1 A/m to 160 A/m, that is, about 0.006 Oe to 20 Oe.
However, a magnetic encoder has a disadvantage that resolution is reduced by only a substitution of the SVGMR elements for AMR elements and coupled GMR elements. When the SVGMR elements are used with a magnetic medium alternately magnetized with N-poles and S-poles with a magnetized pitch λ, a signal has an output cycle of 2λ that is twice of the magnetized pitch. In other words, the resolution becomes a half. This is caused by a magneto-resistance variation characteristic, and the reduction in the resolution cannot be avoided in a conventional encoder structure.
This is because the SVGMR elements have characteristic that electric resistance of the elements changes when an external magnetic field is applied in a same direction as magnetizations of pinned magnetization layers in the elements, while it does not change when the external magnetic field is applied oppositely. Or, because the SVGMR elements have characteristic that electric resistance of the elements does not change when an external magnetic field is applied in a same direction as magnetizations of pinned magnetization layers in the elements, while it changes when the external magnetic field is applied oppositely. When a magnetic medium is magnetized with a magnetized pitch λ, a magnetic field direction changes for every λ. Because of that, electric resistance of SVGMR elements changes with a cycle of 2λ that is twice of the magnetized pitch. By contrast, using coupled GMR elements or AMR elements provides an electric signal of a cycle of λ. The coupled GMR elements and AMR elements show maximum electric resistances in a state of no magnetic field, and the electric resistances reduce when an external magnetic field increases. That is, regardless of a magnetic field direction, a signal is caused by increase and decrease of magnetic field intensity. From the reason, an electric signal of the same cycle as the magnetized pitch λ can be obtained. SVGMR elements have not been applied to a magnetic encoder because they hardly satisfy a high resolution that is demanded in a market. However, since the SVGMR elements show a magneto-resistance variation ratio as much as in coupled GMR elements in a region of a relatively small magnetic field and also an electric resistance as large as five times to six times that of the coupled GMR elements, it is hard to give up an advantage that a low electric power consumption can be easily accomplished by the SVGMR elements.